Many different types of active optical elements for emitting or responding to light used in optical/electrical transducers require effective dissipation of heat. Consider a semiconductor light emitter, such as a light emitting diode (LED) or laser diode, as a first example. To generate more light, the device is driven harder by a higher power drive current. However, the device then generates more heat.
The semiconductor may be damaged or break down if heated to or above a certain temperature. If the temperature gets too high, the device may burn out instantly. All semiconductor light emitters decline in efficiency of light generation as they are operated over time. However, even if the temperature is not high enough to burn out the device quickly, operating a semiconductor light emitter at relatively high temperatures (but below the burn-out temperature) for an extended period will cause the semiconductor light emitter to degrade more quickly than if operated at lower temperatures. Even when a device is running within its rated temperature, the hotter it gets, the less efficient it becomes. Conversely, the cooler the device operates, the more efficient it is.
Many available types of LEDs fail at ˜150° C. LED performance data typically is based on junction temperature of 25° C. However, at more typical junction temperatures (˜100° C.), operating performance is degraded by ˜20% from the specified performance data.
As a solid state light emitter device such as a LED operates, the semiconductor generates heat. The heat must be effectively dissipated and/or the electrical drive power (and thus light output) must be kept low enough, to avoid breakdown or rapid performance degradation and/or to maintain operating efficiency. The package or enclosure of the semiconductor light emitter device typically includes a heat slug of a high thermal conductivity, which is thermally coupled to the actual semiconductor that generates the light. In operation, the slug is thermally coupled to a cooling mechanism outside the device package, such as a heat pipe and/or a heat sink. External active cooling may also be provided.
To increase the intensity of the light generated, the semiconductor light emitter may be driven with a higher intensity electrical current. Alternatively, an overall system or lighting device may include a number of semiconductor light emitters which together can produce a desired quantity of light output. With either approach, the increase in intensity of generated light increases the amount of heat that needs to be dissipated to avoid breakdown or rapid performance degradation and/or to maintain operating efficiency.
Also, many lighting technologies utilize phosphors that are susceptible to overheating. Again, consider a solid state lighting device, for a general lighting application, by way of an example. The solid state light sources typically produce light of specific limited spectral characteristics. To change or enhance the spectral characteristic of a solid state light source, for example, to obtain white light of a desired characteristic, one approach currently favored by LED (light emitting diode) manufacturers, utilizes a semiconductor emitter to pump phosphors within the device package (on or in close proximity to the actual semiconductor chip). Another approach uses one or more semiconductor emitters, but the phosphor materials are provided remotely (e.g. on or in association with a macro optical processing element such as a diffuser or reflector outside the semiconductor package). At least some opto-luminescent phosphors that produce desirable output light characteristics degrade quickly if heated, particularly if heated above a characteristic temperature limit of the phosphor material.
Hence, phosphor thermal degradation can be an issue of concern in many lighting systems. Thermal degradation of some types of phosphors may occur at temperatures as low as 85° C. Device performance may be degraded by 10-20% or more. The lifecycle of the phosphor may also be adversely affected by temperature.
At least some of the recently developed semiconductor nanophosphors and/or doped semiconductor nanophosphors may have an upper temperature limit somewhere in the range of 60-80° C. The light conversion output of these materials degrades quickly if the phosphor material is heated to or above the limit, particularly if the high temperature lasts for a protracted period.
Maintaining performance of the phosphors therefore creates a need for efficient dissipation of any heat produced during light generation. A current mitigation technique for phosphor thermal degradation is to maintain separation of the phosphor from the heat source and maximize unit area of phosphor to minimize flux density. However, the need for more lumens in an output using the phosphor requires larger phosphor unit area, and any limits placed on the flux density to reduce thermal impact on the phosphor constrains the overall device design.
The examples above relate to light generation devices or systems. However, similar heat dissipation issues may arise in devices or systems that convert light to other forms of energy such as electricity. For example light sensors or detectors and/or photovoltaic devices may degrade or breakdown if overheated, e.g. if subject to particularly intense input light of if subject to high light input over extended time periods. Even when a device is running within its rated temperature, the hotter it gets, the less efficient it becomes. Conversely, the cooler the device operates, the more efficient it is.
For these and other types of active optical elements for emitting or responding to light, there is a continuing need for ever more effective dissipation of heat. Improved heat dissipation may provide a longer operating life for the active optical element. Improved heat dissipation may allow a light emitter to be driven harder to emit more light or allow a detector/second or photovoltaic to receive and process more intense light.
Many thermal strategies have been tried to dissipate heat from and cool active optical elements. Many systems or devices that incorporate active optical elements use a heat sink to receive and dissipate heat from the active optical element(s). A heat sink is a component or assembly that transfers generated heat to a lower temperature medium. Although the lower temperature medium may be a liquid, the lower temperature medium often is air.
A larger heat sink with more surface area dissipates more heat to the ambient atmosphere. However, there is often a tension or trade off between the size and effectiveness of the heat sink versus the commercially viable size of the device that must incorporate the sink. For example, if a LED based lamp must conform to the standard form factor of an A-lamp, that form factor limits the size of the heat sink. To improve thermal performance for some applications, an active cooling element may be used, to dissipate heat from a heat sink or from another thermal element that receives heat from the active optical element(s). Examples of active cooling elements include fans, Peltier devices, membronic cooling elements and the like.
Other thermal strategies for equipment that use active optical elements have utilized heat pipes or other devices based on principles of a thermal conductivity and phase transition heat transfer mechanism. A heat pipe or the like may be used alone or in combination with a heat sink and/or an active cooling element.
A device such as a heat pipe relies on thermal conductivity and phase transition between evaporation and condensation to transfer heat between two interfaces. Such a device includes a vapor chamber and working fluid within the chamber, typically at a pressure somewhat lower than atmospheric pressure. The working fluid, in its liquid state, contacts the hot interface where the device receives heat input. As the liquid absorbs the heat, it vaporizes. The vapor fills the otherwise empty volume of the chamber. Where the chamber wall is cool enough (the cold interface), the vapor releases heat to the wall of the chamber and condenses back into a liquid. Thermal conductivity at the cold interface allows heat transfer away from the mechanism, e.g. to a heat sink or to ambient air. By gravity or a wicking structure, the liquid form of the fluid flows back to the hot interface. In operation, the working fluid goes through this evaporation, condensation and return flow to form a repeating thermal cycle that effectively transfers the heat from the hot interface to the cold interface. Devices like heat pipes can be more effective than passive elements like heat sinks, and they do not require power or mechanical parts as do active cooling elements. It is best to get the heat away from the active optical element as fast as possible, and the heat pipe improves heat transfer away from the active optical element, even where transferring the heat to other heat dissipation elements.
Although these prior technologies do address the thermal issues somewhat, there is still room for further improvement.
For example, passive cooling elements, active cooling elements and heat transfer mechanisms that rely on thermal conductivity and phase transition have been implemented outside of the devices that incorporate active optical elements. A light processing device may include one or more elements coupled to the actual active optical element to transfer heat to the external thermal processing device. In our LED example, heat passes through of the layers of the semiconductor, to the heat slug and then to the external thermal processing device(s). The need to transfer the heat through so many elements and the various interfaces between those elements reduces efficiency in cooling the thermally susceptible component(s) of the active optical element. Again referencing the LED example, the need to transfer the heat through so many elements reduces efficiency in cooling the LED chip, particularly cooling at the internal the layer/point in the semiconductor chip where the light is actually generated.
It has been suggested that a heat pipe type mechanism could be incorporated at the package level with the LED (WO 2007/069119 (A1)). However, even in that device, a heat spreader and a light transmissive collimator encapsulate the actual LED chip and separate the chip from the working fluid. Heat from the LED chip structure is transferred through the heat spreader to the working fluid much like the prior examples that used an external heat pipe coupled to the heat slug of the LED package.
There is an increasing desire for higher, more efficient operation (light output or response to light input) in ever smaller packages. As outlined above, thermal capacity is a limiting technical factor. Thermal capacity may require control of heat at the device level (e.g. transducer package level and/or macro device level such as in a lamp or fixture). Also, for equipment utilizing phosphors, there is a continuing need for ever more effective dissipation of heat. Improved heat dissipation may provide a longer operating life for the apparatus or device using the phosphor(s). Improved heat dissipation may allow a device to drive the phosphor harder, to emit more light, for a particular application.
Hence, it may be advantageous to reduce the distance and/or number elements and interfaces that the heat must pass through from the active optical element. As outlined above, thermal capacity may require control of heat at the phosphor level. Hence, it may be advantageous to improve technologies to more effectively dissipate heat from and/or around phosphor materials. Also, improvement in technologies to more effectively dissipate heat from active optical elements may help to meet increasing performance demands with respect to the various types of equipment that use the active optical elements.